Data transmission method with a level of error protection selected from among several pre-defined levels

ABSTRACT

The invention relates to a method whereby the control unit produces a sequence of blocks, each comprising a header and data to be transmitted. Each block is transmitted to the destination terminal with a level of error protection which is selected from among several pre-defined levels, the selected level being indicated in a piece of signalling information that accompanies the block transmitted. The header of each block comprises an acknowledgement control field which is activated intermittently by the control unit in order to request an acknowledgement of blocks from the terminal. A higher level of protection is selected for transmitting blocks having a header with an activated acknowledgement control field than for transmitting blocks having a header with a non-activated acknowledgement control field.

BACKGROUND OF THE INVENTION

The present invention relates to techniques for transmitting packets inacknowledged mode from a master control unit to a slave terminal.

The invention relates more particularly, among these techniques, tothose in which the control unit, which produces a sequence of blockseach comprising a header in addition to the user data, uses a “polling”mechanism to request acknowledgement of packets from the remoteterminal. The header of each block then comprises an acknowledgementcontrol field activated intermittently by the control unit so as torequest acknowledgement of blocks from the terminal. In response to sucha request, the terminal returns a message in which a certain number ofprevious blocks are acknowledged, positively and/or negatively.

A technique of this kind is used, in particular, for the downlinks (fromthe network to the mobile terminals) in GPRS (“General Packet RadioService”) networks that have been developed to allow the transmission ofdata in packet mode in GSM (“Global System for Mobile communications”)type cellular networks.

The mechanism for acknowledging the packets may be disturbed when ablock whose header has an activated acknowledgement control field ispoorly received by the slave terminal. In this case, the terminal doesnot execute the acknowledgement request, so that the control unitremains uncertain as to the blocks that have been correctly received.

Often, the acknowledgement mechanism is used within the framework of anautomatic repeat protocol (ARQ, “Automatic Repeat reQuest”) in which theunit sending the packets uses a send window of specified length,positioned onward of the first block which has not yet been positivelyacknowledged. If an acknowledgement request message has not beencorrectly received by the remote terminal, the send window may remainblocked at an old position, thereby leading to unnecessary repetitionsof already received packets and to significant delays in thetransmission of the new packets.

An object of the present invention is to propose an efficient method fortransmitting data in packet mode.

Another object is to reduce the risks of blocking of the send windowsutilized in certain ARQ mechanisms.

SUMMARY OF THE INVENTION

The invention thus proposes a method for transmitting data inacknowledged mode between a control unit and a terminal, wherein thecontrol unit produces a sequence of blocks each comprising a header anddata to be transmitted, and the blocks are transmitted to the terminal,each block being transmitted with a level of protection against errorsselected from several predefined levels, the selected level beingindicated in signaling information accompanying the transmitted block.The header of each block comprises an acknowledgement control fieldactivated intermittently by the control unit so as to request anacknowledgement of blocks from the terminal. According to the invention,a higher level of protection is selected for the transmission of atleast one block whose header has an activated acknowledgement controlfield than for the transmission of the blocks the acknowledgementcontrol field of whose header is not activated.

The process uses differentiated protection of the transmitted blocks,depending on whether or not they contain an acknowledgement request. Thebetter protection of the acknowledgement requests, which are transmittedin the band with the user data, makes it possible to avoid a largeproportion of the blockages that acknowledgement mechanisms might giverise to.

Most often, protection against transmission errors is ensured by achannel coder using a convolutional code or a block code. Differentlevels of protection are then effected by adjusting the coding rate:introduction of additional redundancy symbols, modification of thestructure of the code used, alteration of the degree of puncturing ofthe code, etc.

Other methods may be used to adapt the level of protection againsterrors, for example methods of adaptive control of the signalstransmission power.

Another aspect of the present invention concerns a packet control unit,comprising means of producing at least one sequence of blocks eachcomprising data to be transmitted and a header including anacknowledgement control field, means for transmitting the blocks of thesequence to a terminal, means of selecting a level of protection againsterrors for the transmission of each block of the sequence, from severalpredefined levels, and means of intermittent activation of theacknowledgement control field in the header of the blocks of thesequence so as to request an acknowledgement of blocks from the terminalto which the block is transmitted. The means of selection are devised toselect a higher level of protection for the transmission of at least oneblock whose header has an activated acknowledgement control field thanfor the transmission of the blocks the acknowledgement control field ofwhose header is not activated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a GPRS-type network to which the invention may beapplied;

FIG. 2 is a schematic diagram of a packet control unit of such anetwork, adapted to the implementation of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The GPRS network illustrated in FIG. 1 is built on a GSM infrastructure,and conventionally divided into a network core, also called Network andSwitching Subsystem or NSS, and a radio-access network also called BaseStation Subsystem or BSS.

For the packet service, the switches of the NSS are called GPRS supportnodes or GSNs. A distinction is made between the SGSNs (Serving GSNs) 5,which are linked to the BSS by way of an interface called Gb, and theGGSNs (Gateway GSNs, not represented) which serve as a gateway withexternal packet transmission networks, such as the Internet, forexample.

A general description of the radio interface, called Um, between themobile stations (MS) 10 and the base stations (BTS) 20 of the BSS isprovided in the technical specification ETSI TS 101 350, “Digitalcellular telecommunications system (Phase 2+); General Packet RadioService (GPRS); Overall description of the GPRS radio interface; Stage 2(GSM 03.64, version 6.3.0, Release 1997)”, published by ETSI (EuropeanTelecommunications Standards Institute) in July 1999.

Each base station 20 is supervised by a base station controller or BSC21 by way of an interface called Abis. In order to manage thetransmission of GPRS packets, the BSS further comprises an entity 22called packet control unit or PCU. The locating of the PCU within theBSS is not standardized. In the example represented in FIG. 1, the PCU22 is situated between the BSC 21, with which it communicates via aninterface called Agprs, and the NSS, with which it communicates via theinterface Gb.

FIG. 2 illustrates a possible structure of a PCU 22 situated between anSGSN 5 and a BSC 21, as in the example of FIG. 1. The reference 40designates the Gb interface controller for the link with the SGSN 5.

The Gb interface is of asynchronous type. It is based on the frame relay(FR) protocol, as well as on a protocol called BSSGP (BSS GPRSProtocol.) which transports routing and quality-of-service informationbetween the BSS and the SGSN. The Gb interface controller 40 providesthe physical link with the SGSN 5, as well as carrying out theprocedures specific to the FR and BSSGP protocols.

The links between the PCU 22 and the BTSs 20 via the Agprs interface areof synchronous type. Consequently, the data manipulated by the PCU 22between the Gb interface controller 40 and the Agprs interfacecontroller 42 transit via a buffer memory 41 where packet queues arerecorded.

Between the PCU 22 and the BTS 20, the information is carried by 320-bitframes of TRAU (Transcoder/Rate Adapter Unit) type, at the rate of oneframe every 20 ms. These TRAU frames are formatted and processed by amodule 44 and transmitted by way of synchronous interface circuits 45which form MIC subchannels at 16 kbit/s with the BTSs 20. Several16-kbit/s subchannels can be multiplexed on the Agprs interface andseparated by the BSC 21 for routing to the BTSs. A module 46 of theAgprs interface controller 42 implements the radio protocols of layer 2of the OSI model, that is to say the RLC/MAC (Radio Link Control/MediumAccess Control) protocols described in the European Standard ETSI EN 301349, “Digital cellular telecommunications system (Phase 2+); GeneralPacket Radio Service (GPRS); Mobile Station (MS)—Base Station System(BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC)protocol (GSM 04.60, version 6.8.1, Release 1997)”, published by ETSI inOctober 2000.

The RLC sublayer forms the interface with the upper-layer protocol,called LLC (Logical Link Control). It carries out the segmentation andthe reassembling of LLC protocol data units (LLC-PDUs), which areexchanged asynchronously on the Gb interface. It produces RLC datablocks to which the MAC sublayer adds a one-byte MAC header.

In the downlink direction, from the PCU to the MSs, the MAC header ofeach RLC/MAC block includes:

-   -   a three-bit USF (Uplink State Flag) field, serving to indicate        which mobile station is authorized to use an uplink resource        corresponding to the downlink resource on which the RLC/MAC        block is transmitted;    -   a three-bit acknowledgement control field, including a one-bit        S/P (Supplementary/Polling) subfield indicating whether the        acknowledgement control field is active (S/P=1) or inactive        (S/P=0) and a two-bit RRBP (Relative Reserved Block Period)        subfield uniquely specifying an uplink block in which the mobile        station addressed should transmit an acknowledgement message;    -   a two-bit Payload Type field, specifying the type of RLC block        following (data, control, etc).

It is the transmission of RLC data blocks which is of interest here.Each of these blocks includes an RLC header following the MAC headerbyte. This RLC header especially includes the following information:

-   -   temporary flow identity (TFI), consisting of five bits        identifying the temporary block flow (TBF), from which the RLC        data of the block originate. A TBF is a connection supporting        the unidirectional LLC-PDU transfer on physical data channels. A        TBF is temporary, that is to say that it is maintained only        during the data transfer;    -   a block sequence number BSN of seven bits, which contains the        sequence number of the RLC/MAC block, modulo 128.

The MAC sublayer furthermore manages the multiplexing of the blocksarising from the various TBFs which are active on the available physicalchannels, arbitrating among the various mobile users via a planningmechanism (“scheduling”).

The RLC/MAC entity of the destination mobile station receiving thedownlink data blocks from a TBF updates for this flow a reception statevariable V(R) which indicates the BSN following the highest BSN receivedon this TBF. The number V(R)-1 (modulo 128) thus points to the end of areception window whose length is k=64 RLC/MAC blocks. On receipt of a“polling” command, that is to say of a block whose MAC header has thebit S/P=1, the MS returns in the uplink block specified by the RRBPsubfield, an acknowledgement message PDAN (“Packet Downlink Ack/Nack”)which comprises in particular:

-   -   a field SSN (“Starting Sequence Number”) of seven bits        containing the current variable V(R) for the TBF; and    -   a field RBB (“Receive Block Bitmap”) of k=64 bits indicating        those of the blocks of the reception window that have been        correctly received. A positive acknowledgement of the block        BSN=(SSN-i) mod 128 is indicated by the value 1 of the bit of        rank i (1≦i≦k) of the RRB bitmap, and a negative acknowledgement        by the value 0.

On receipt of the PDAN message, the PCU updates for the TBF anacknowledgement state variable V(A) which contains the BSN of the oldestblock that has not been positively acknowledged, as well as a table V(B)with k inputs indicating the respective acknowledgement states (positiveacknowledgement/negative acknowledgement/acknowledgement not received)of k consecutive blocks following the one designated by V(A), these kconsecutive blocks forming a send window. The state variables V(A) andV(B) are deduced directly from the SSN and RBB fields received in thelast PDAN message. The RLC/MAC protocol does not authorize thetransmission of blocks other than inside the send window thus managed bythe PCU. Outside of this window, the transmission of the blocks isinhibited.

So as not to delay the transmission of the new blocks, it is advisableto avoid the phenomena of blocking of the send window on an obsoleteposition. Such blocking may in particular occur when errors affect thetransmission of the “polling” commands sent by the PCU. To avoid this,it is proposed that a protection specific to the downlink data blockswhose MAC header has the bit S/P=1 be applied.

In the case of the GPRS, a variable level of protection can be selectedblock by block within a TBF, by the choice of a coding scheme (CS) fromamong four schemes CS-1 to CS-4 specified in the European Standard ETSIEN 300 909, Digital cellular telecommunications system (Phase 2+);Channel coding (GSM 05.03, version 6.2.1, Release 1997), published byETSI in August 1999.

The scheme CS-4 does not use any error-correction coding, that is to saythat the coding rate is equal to 1: only a block check sequence BCS isadjoined to the data blocks. The schemes CS-1 to CS-3 use aconvolutional code with rate ½ after the addition of the BCS sequence.No puncturing is carried out in the

CS-1 scheme (which offers the highest level of protection), whilepuncturing is applied in the CS-2 and CS-3 schemes so that they giverise to overall coding rates of about ⅔ and of about ¾, respectively.

The CS-1 (1≦i≦4) channel coding is applied at the level of thephysical-layer protocol, that is to say in the BTSs in the case of thedownlinks. Each coded RLC/MAC block is composed of 456 bits and istransmitted in corresponding time intervals of four TDMA frames on acarrier frequency, the successive TDMA (“Time-Division Multiple Access”)frames each being split into eight time intervals to ensuretime-division channel multiplexing.

A pattern of eight signaling bits is inserted into each coded frame (twobits per time interval) so as especially to indicate which coding schemehas been applied by the transmitter.

These signaling bits are extracted from the coded block received by theaddressee, in order to allow it to identify the coding scheme. Thereceiver then carries out the appropriate decoding of the block whichwill give rise to a positive acknowledgement if it is successful and ifthe decoded BCS is consistent with the content of the block.

The coding scheme applied to the downlink is determined in a way whichis known in itself by the PCU on the basis of measurements of receptionquality on the radio link, according to link-adaptation mechanisms whichseek to achieve an objective in terms of rate of error-affected blocksso as to optimize the raw throughput. The scheme selected is insertedinto the TRAU frame carrying the block so as to be applied by the BTS.

Each time a coding scheme other than CS-1 is determined by the linkadaptation mechanisms, the RLC/MAC layer selects a scheme which is morerobust to errors for each block whose MAC header has the bit S/P=1. Inparticular the CS-1 scheme (maximum level of protection) may be adoptedsystematically for these blocks which contain acknowledgement requests.

It should be noted that this systematic selection of the CS-1 schememight not be applied when the TBF is in the termination phase and whenall the RLC data of this TBF have been sent at least once. Specifically,the 1997 release of the RLC/MAC protocol of GPRS prescribes theresending of the data in the same code as the first send. For example,if only blocks already sent in CS-4 are to be resent in the terminationphase of the TBF, the repetitions will also be coded in CS-4 even iftheir MAC header contains S/P=1. However, certain link adaptationmechanisms tend to favor a more robust coding scheme at the end of aTBF, so that the risks of window blocking remain small.

1. A method for transmitting data in acknowledged mode between a controlunit and a terminal, wherein the control unit produces a sequence ofblocks each comprising a header and data to be transmitted, and theblocks are transmitted to the terminal, each block being transmittedwith a level of protection against errors selected from severalpredefined levels, the selected level being indicated in signalinginformation accompanying the transmitted block, wherein the header ofeach block comprises an acknowledgement control field activatedintermittently by the control unit so as to request an acknowledgment ofblocks from the terminal, wherein a higher level of protection isselected for the transmission of at least one block whose header has anactivated acknowledgment control field than for the transmission of theblocks the acknowledgement control field of whose header is notactivated.
 2. The method as claimed in claim 1, wherein a maximum levelof protection is selected for the transmission of the block whose headerhas an activated acknowledgement control field.
 3. The method as claimedin claim 1, wherein the transmission of the blocks to the terminal isinhibited outside of a window composed of a specified number of blocksand whose position in the sequence is determined by the control unit onthe basis of acknowledgment information received from the terminal inresponse to the blocks whose header has an activated acknowledgmentcontrol field.
 4. The method as claimed in claim 1, wherein thepredefined levels of protection against errors result from a channelcoding of adjustable rate to which the blocks are subjected.
 5. A packetcontrol unit, comprising means of producing at least one sequence ofblocks each comprising data to be transmitted and a header including anacknowledgement control field, means for transmitting the blocks of thesequence to a terminal, means of selecting a level of protection againsterrors for the transmission of each block of the sequence, from severalpredefined levels, and means of intermittent activation of theacknowledgment control field in the header of the blocks of the sequenceso as to request an acknowledgment of blocks from the terminal to whichthe block is transmitted, wherein the means of selection are devised toselect a higher level of protection for the transmission of at least oneblock whose header has an activated acknowledgment control field thanfor the transmission of the blocks the acknowledgment control field ofwhose header is not activated.
 6. The control unit as claimed in claim5, wherein the means of selection are devised to select a maximum levelof protection for the transmission of the block whose header has anactivated acknowledgment control field.
 7. The control unit as claimedin claim 5, wherein the transmission of the blocks to the terminal isinhibited outside of a window composed of a specified number of blocksand having a position in the sequence determined on the basis ofacknowledgment information received from the terminal in response to theblocks whose header has an activated acknowledgment control field.